The present invention relates to the general field of diffusers for gas turbine engines of terrestrial or aviation type. It relates more particularly to diffusers placed between the turbine and the exhaust casing of a gas turbine engine.
The function of terrestrial or aviation gas turbines is to deliver power that is sufficient to drive either an alternator (terrestrial turbines) or a compressor (aviation turbines). To do this, a gas turbine takes a fraction of the energy of the hot compressed gases coming from the combustion chamber of the turbine engine and transforms it into mechanical energy. A turbine generally comprises a plurality of stages, each stage comprising a stator nozzle and a moving wheel placed after the nozzle for accelerating the flow of gas. The gas coming from the last stage of the turbine then feeds an exhaust casing.
The exhaust casing placed immediately downstream from the turbine is constituted by a diffuser and by casing arms which serve essentially to straighten the flow of gas at the outlet of a non-axial turbine and to pass cooling air for the internal portions of the engine. The diffuser serves to reduce the speed and increase the pressure of the gas coming from the last stage of the turbine. For this purpose, the diffuser generally comprises walls forming a passage for the gas, which walls diverge in the gas flow direction, as shown in U.S. Pat. No. 2,594,042.
An exhaust casing suffers from pressure losses which are typically proportional to the square of the speed of the gas at the leading edge of the casing arms. For example, for a terrestrial turbine, the gas reaches a speed close to Mach 0.6 at the outlet from the moving wheel of the last stage of the turbine. The diffuser enables this speed to be reduced to about Mach 0.45 at the leading edge of the casing arms, which leads to pressure losses of about 5%. Nevertheless, a gas speed of about Mach 0.45 still constitutes a value that is high. The slope of the walls constituting the diffuser must not exceed a certain value since otherwise there is a risk of boundary layers on said walls thickening. Thick boundary layers lead to separation, which harms the efficiency of the diffuser. Thus, when separation from the walls of the diffuser occurs, the aerodynamic section downstream therefrom is much smaller than its geometrical section, thus preventing the diffuser from performing its diffusion function. Furthermore, optimizing the turbine in terms of cost, mass, and performance generally leads to high loads per stage, giving rise to ever-increasing speed of the gas at the outlet from the last stage of the turbine.